1. Field of the Invention
The present invention relates to a wavelength stabilized light source. The present invention relates particularly to a wavelength stabilized optical transmitting apparatus and an optical transmitting apparatus both having a plurality of light sources, wherein each of wavelengths employed in a wavelength division multiplexing optical communication system is controlled to a predetermined value.
2. Description of the Related Art
With abrupt growth of the recent information communication market, an optical transmitter-receiver apparatus or system capable of transmitting a larger quantity of information has actively been developed. The development of a wavelength division multiplexing optical transmitter-receiver system capable of making a leap increase in the information amount per optical fiber becomes pronounced in particular. However, since a limitation is imposed on a wavelength region suitable for transmission executed with a transmission loss less reduced at each optical fiber. It is necessary to increase a number of optical channels for optical or light signals within the limited wavelength region. It is necessary to narrow the interval between adjacent wavelengths of each individual light signals for the purpose of increasing the number of optical channels. A problem arises in that a semiconductor laser principally used as a light source for a conventional wavelength division multiplexing optical transmitting apparatus or system greatly varies in wavelength according to changes in ambient temperature and drive current.
Further, a phenomenon has widely been known wherein even if the ambient temperature of the semiconductor laser is kept constant and the drive current is also held constant, the wavelength varies due to a variation with time such as a change in composition of the semiconductor laser if it is used over a long period. There is a possibility that such a variation in wavelength from each set value will cause interference from other light signals, a substantial degradation in transmission quality or a fall into an impossible-to-transmit state. It is therefore essential that an optical transmitter is provided with some wavelength monitoring mechanism and wavelength tunable mechanism to perform control for correcting a shift or deviation made from each set wavelength with a view toward implementing a high-density wavelength division multiplexing optical transmission system capable of narrowing a wavelength-to-wavelength interval.
As the optical transmitting apparatus provided with the wavelength monitoring mechanism and the wavelength tunable mechanism, an optical wavelength division multiplexing transmitting apparatus shown in FIG. 9 is known (see Japanese Patent Application Laid-Open No. Hei 10-209973, for example). Namely, optical or light signals 9 through 12 having wavelengths xcex1, xcex2, xcex3 and xcex4 different from one another, which are sent from optical transmitters 1 through 4, are respectively applied to an optical multiplexer or an optical coupler 300 through optical fibers 5, 6, 7 and 8, where they are multiplexed into a wavelength division-multiplexed signal 310, which in turn is applied to an optical power divider 305 through an optical fiber 304. The wavelength division-multiplexed signal 310 is divided into two by the optical power divider 305, one of which is sent to a wavelength deviation detector 302 as a wavelength division-multiplexed signal 311 through an optical fiber 307, and the other of which is transmitted through an optical fiber 306.
The wavelength deviation detector 302 generates deviations equivalent to differences between values of wavelengths included in the wavelength division-multiplexed signal 311 and the set values of the wavelengths assigned to the optical transmitters in advance, and sends their deviation signals 313 to a wavelength control circuit 301 on a time-sharing basis. The wavelength control circuit 301 and the optical transmitters 1 through 4 are respectively connected to one another through wires 321 and 324. The wavelength control circuit 301 generates wavelength control signals 331 through 334 for controlling the values of the wavelengths of the present signals to the values of the wavelengths assigned in advance, based on the respective deviation signals inputted to the wavelength control circuit 301. For example, signals for controlling the temperatures of light sources in the optical transmitters 1 through 4 or signals for controlling drive currents of the light sources are used as the wavelength control signals 331 through 334. The wavelength control signals 331 through 334 are distributed to the optical transmitters 1 through 4 through the wires 321 through 324 respectively. The optical transmitters 1 through 4 respectively control the wavelengths of their own light sources according to the wavelength control signals 331 through 334 and send out the wavelengths assigned to their own optical transmitters in advance.
A mechanism for identifying each individual light signals included in wavelength division-multiplexed light signals and measuring the values of wavelengths of the light signals is essential to the aforementioned prior art.
For example, intensity modulation is slightly effected on each of light signals sent out from within each individual optical transmitters, based on low-frequency signals different from an information transmission region. The values of the frequencies of these low-frequency signals are respectively set to different values every optical transmitters, and a wavelength deviation detector performs phase sensitive detection with the frequencies of the respective low-frequency signals, whereby deviations corresponding to the differences between the wavelengths of each individual optical transmitters and the wavelengths assigned to the optical transmitters in advance can be detected. However, a problem arises in that the intensity modulation using such low frequencies is unnecessary for original information transmission and might be one cause of degradation in transmission quality.
As a mechanism for identifying other light signals and measuring wavelengths thereof, a system is also known wherein a wavelength deviation detector measures the wavelength of each light signal while successively scanning a band-pass wavelength variable filter or an optical power divider or the like, and measures deviations corresponding to the differences between the measured wavelengths of light signals and the wavelengths assigned to each individual optical transmitters in advance. However, a problem arises in that since no identifying information exists in each light signal itself, it is not possible to correct each wavelength when the wavelength of the optical transmitter 1, which is to be originally set as the wavelength xcex1, and the wavelength of the optical transmitter 2, which it to be originally set as the wavelength xcex2, are respectively replaced by xcex2 and xcex1 with respect to each other.
Since, in either case, all the optical transmitters and the wavelength deviation detector are elements which constitute an electrical control loop, there is a possibility that when one optical transmitter is replaced by another due to it trouble or malfunction, it will interfere with the operation of other normal optical transmitters.
Thus, a principal object of the present invention is to implement an optical transmitting apparatus capable of stabilizing wavelengths of each individual optical transmitters without superimposing signals other than information necessary for information transmission, i.e., signals dedicated for wavelength control on light signals respectively.
Another object of the present invention is to implement an optical transmitting apparatus capable of achieving the above object, and when a failure or malfunction occurs in any of respective optical transmitters, performing such wavelength control as not to exert an influence on other optical transmitters due to the replacement of the faulty optical transmitter by another.
In an optical transmitting apparatus of the present invention to achieve the above objects, control units for receiving other light signals used to control characteristics such as wavelengths, optical intensities, optical phases, etc. of light signals respectively sent from optical transmitters, from the outside the optical transmitters and controlling the respective optical transmitters based on the received signals are respectively provided inside the optical transmitters or provided adjacent to the optical transmitters. Namely, a reference light source (wherein the wavelengths of signals generated therefrom are different from those of the light signals transmitted from the optical transmitters. Further, the wavelengths of the generated light signals are regarded as sufficiently stable.) capable of simultaneously or successively generating a plurality of reference wavelength light signals, means for demultiplexing the respective reference wavelength light signals emitted from the reference light-source and inputting the demultiplexed reference wavelength light signals to the plurality of optical transmitters respectively, and a control unit for controlling light-source wavelengths so as to allow light-source wavelengths of the optical transmitters to coincide with the wavelengths of the reference wavelength light signals, corresponding to the corresponding optical transmitters, of the input respective reference wavelength light signals, or so as to reach inherent or proper values at which differences in wavelength exist, are provided for each optical transmitter.